The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the measurement and correction of geometric distortion in MRI images due to magnetic field non-linearities and inhomogeneities.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) are employed to locate the position of the excited spins. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The reconstructed MRI image can be geometrically distorted due to a number of causes. The location of an object, or portions of the object, can be shifted due to changes in the local Larmor resonance frequency. Chemical shift artifacts caused by small differences in Larmor resonance frequency (e.g. fat and water spins), susceptibility differences between different tissues, and patient induced or polarizing magnetic field inhomogeneities are examples of sources for such shifting of position. Another source of distortion is stretching or contracting of the imaged object caused by non-linearity of one or more of the applied gradient magnetic fields. This distortion is referred to as "warping". Both distortions can be very substantial, however, in standard cylindrical magnets used for MR imaging and spectroscopy, warping distortion is usually much greater than the distortion due to shifting. This is particularly true when local gradient coils are employed in lieu of whole body gradient coils, since the field of view over which local coil gradient fields are linear is very limited. In addition to these "static" geometric distortions, time variant and spatially variant gradient eddy currents also contribute to both object shifting and image warping.
Techniques such as those disclosed in U.S. Pat. Nos. 4,591,789 and 4,672,320 have been developed to correct magnitude MRI images for these various types of geometric distortions. These corrections are based on mathematical models of the gradient fields produced by the gradient coil structures. While these models are generally accurate, they do not reflect variations in gradient coil structures due to manufacturing tolerances and changes that may occur in the gradient coil structures during use.